Low-density polyethylene and copolymers of ethylene with polar co-monomers can be produced by free radical polymerization at high temperature (e.g., 100-350° C.) and ultra-high pressure (e.g., 10,000-50,000 psig). These reactions are often carried out in a thick-walled autoclave reactor specially designed to withstand the extreme operating conditions.
To control temperature and polymer concentration, intense agitation to establish uniformity throughout the reactor is required. If regions within the reactor are insufficiently agitated, polymerization in such localized zones can rise above that in the rest of the vessel, and the temperature can rise in those areas to form hot spots and cause runaway decompositions.
To improve mixing, radial-impeller stirrers have been developed. These stirrers perform better than the prior generation of stirrers, which contain a large number of agitating paddles. However, users of the newer stirrers are still experiencing a high tendency of runaway decompositions, especially in reactors larger than 250 liters (L) and in those operating at hot top-zone conditions (e.g., >200° C.).
It has been surprisingly discovered that these newer stirrers provide too little axial and radial flow and mixing, despite providing quite a high rotational flow.
One potential solution to this problem is to employ wall baffling. However, with an ultra-high pressure autoclave, there are important mechanical reasons to avoid welding and thread-bolting appurtenances to the wall of the main cylindrical body. For example, at 3200 psig, the steel used to make the reactor wall can be welded without fear of developing stress cracks. On the other hand, at 10,000 psig, the conditions are much more severe, and the potential of developing stress cracks is very real. The mechanical stresses are very high on the main cylinder wall of a commercial scale autoclave at pressures greater than 10,000 psig. Indeed, the material of choice is typically a steel alloy with unusually high tensile strength in order to cope with the extreme pressure. Unfortunately, such steels have a greater tendency to crack.
Generally, all bores into the cylinder wall go completely through and are carefully chamfered and polished at the inside surface in order to remove stress risers that would serve as crack initiation points. In addition, the vessel must also undergo periodic stripping and crack-detection inspections. Thus, attaching a wall baffle by threading a stud or bolt to the inside surface of the cylinder is not considered to be a viable option, since even rolled threads (contrast to cut threads) pose too much cracking risk and inspection difficulty. Similarly, welding a wall baffle to the inside surface of the cylinder would degrade the integrity of the quenched and forged metal, making it either weaker or harder, and thus more prone to cracking.
Accordingly, there is a need in the art to improve mixing in an autoclave reactor for ultra-high pressure ethylene polymerization processes in order to suppress decompositions and to improve product properties. The present invention is directed to addressing this need as well as others that will become apparent from the following description and the appended claims.